System and method for performing well operations

ABSTRACT

System and associated method for performing well operations in a plurality of hydrocarbon wells ( 120   a   , 120   b   , 120   c ) of a wellhead cluster ( 104 ), has a main drilling rig ( 106 ) with a main drilling unit ( 107 ) displaying a drill floor ( 108 ) and having a drilling and hoisting system ( 109 ) arranged to operate on the drill floor about a well center ( 110 ), wherein the main drilling unit is positioned at least partially vertically above the wellhead cluster enabling the drilling and hoisting system to perform well operations in the plurality of hydrocarbon wells. A plurality of secondary drilling units ( 105   a   , 105   b ), each having its own drilling equipment for performing drilling operations, are positioned on the wellhead cluster below the main drilling unit for performing well operations in the plurality of hydrocarbon wells, wherein the main drilling unit and at least one of the plurality of secondary drilling units are arranged for performing alternating well operations in at least one of the plurality of hydrocarbon wells.

FIELD OF THE INVENTION

The present invention relates to a method and a system for performingseveral well activities, comprising a main platform with a main drillingunit with a drill floor and equipped with several distributed wellcenters on a wellhead spread.

In particular, the invention relates to a system for performing welloperations in a plurality of hydrocarbon wells of a wellhead cluster,which system comprises a main drilling rig comprising a main drillingunit displaying a drill floor and comprising a drilling and hoistingsystem arranged to operate on the drill floor about a well center,wherein the main drilling unit is positioned at least partiallyvertically above the wellhead cluster enabling the drilling and hoistingsystem to perform well operations in the plurality of hydrocarbon wells.

The invention also relates to a related method of performing welloperations in a plurality of hydrocarbon wells of a wellhead cluster.

In this application, the term “wellhead cluster” is used to denote aplurality of wellheads which are located on a common wellhead template,e.g. on a wellhead jacket platform or a wellhead spread, or within alimited geographical area without necessarily being located on a commonwellhead template.

BACKGROUND OF THE INVENTION

This invention relates to a novel method for well operations. Morespecifically, this invention relates to a method and apparatus forconducting simultaneous operations on several wells by utilizing onemain drilling unit with several secondary drilling units, also known as“drilling pods”, positioned on distributed well centers. This can alsobe defined as Multi-well drilling, from one drilling unit.

For development drilling, multiple wells will normally be drilled fromone platform or area. On shallow water drilling, wellhead jacketplatforms are normally used. For some wellhead jackets it is normal touse a drilling module placed on an x-y skid module for moving betweenthe different wells. For wellhead jacket platforms without drillingmodule or if the platform drilling module is removed after the wells arefinished, or the drilling module is not fit for further use due todifferent reasons (regulations, certification, low efficiency), the useof a main drilling rig, such as a jack-up rig, may be a solution forfurther drilling, workover or plugging the wells on this platform.

This invention is intended to increase the efficiency of the work bybeing able to work on several wells at the same time by use of one maindrilling unit positioned on an elevated drilling floor of the maindrilling rig and one or several smaller secondary drilling units, or“drilling pods”, positioned on the wellhead jacket platform, and by thismethod reduce the time and cost for each well.

This invention uses two or more separate and independent secondarydrilling units, which will also be referred to as “drilling pods” in thefollowing description, comprising short hoisting masts situated on thedifferent wells supported by a main drilling unit with an elevated drillfloor such as a jack-up rig. The elevated drill floor can skid above thedrilling pods and assist to improve efficiency or increase capacity ofsome operations like; trip in and out with drill pipe, run or pull largecasings/tubings/conductors. The elevated drill floor can be equippedwith large storage capacity and allow for offline activities.

A large main drilling rig, e.g. a jack-up rig with drill floor on thecantilever or an elevated drilling module, with reach to all or aportion of the well slot positions, is required to enable increasedefficiency for the overall operation. The main drilling unit can handlethe installation, and support the operation of the secondary drillingunits or “drilling pods” with; accommodation, power, air, water,hydraulics, mud, cement, cuttings handling, bop control fluid and drainfluid handling in addition to tubular and well tools logistics. Thedrilling control system for the elevated main drilling rig can bearranged to take over the control for each secondary drilling unit or“drilling pod”. The elevated drill floor of the main drilling rig canhave separate operation rooms from where the support of the secondarydrilling units, or “drilling pods”, can be coordinated. Servicecompanies can assist the simultaneous well operations from the drillingareas on the drilling unit or the wellhead deck or surroundings.

Each of the secondary drilling units, or “drilling pods”, can have itsown driller/operator cabin, where full control of the operation shall bepossible. The secondary drilling units can be made such as they areoptimized for their intended purpose with an optimal footprint, heightand weight allowing close by operations as well as easy mobilization bythe main drilling unit on to the wellhead deck. Only a few lifts can berequired to enable fast rig up of the secondary drilling units. Thesecondary drilling units can be able to trip to the specified depth,drill, ream, circulate and set plugs as a normal drilling rig as well asfacilitating for other well operations such as wireline, coiled tubing,snubbing, etc. The BOP can be placed in the substructure of thesecondary drilling units, and the kill and choke lines can be connectedback to the main drilling unit by high pressure hoses. The main drillingunit as well as the secondary drilling units shall meet governmentalregulations and international regulations for the equipment.

The above described main drilling unit and the secondary drilling units,or “drilling pods”, can be able to skid independently between well slotsand work independently in a planned pattern to obtain high efficiencyfor the work. By planning the well sequence, it can be possible thatonly one drilling unit at the time is working in the reservoir section.For some periods the main drilling unit can perform well operationsadjacent to the secondary drilling units such as pulling and laying outconductors, which has already been cut below ground. At the same timethe two or more secondary drilling units can be working on other wells.Even with two secondary drilling units this enables up to three welloperations at the same time supported from one main drilling rig.

It may also be possible to initiate other well operations such aswireline operations or coiled tubing operations on wells not beingoperated on by the second drilling units, or “drilling pods”, in orderto perform well killing, intervention or similar. If the main drillingrig needs repositioning to reach all wells on the wellhead spread, suchoperations may provide operational progress on the wells program untilthe weather allows the moving of the main drilling rig to a new positionwith an extended coverage of the wellhead spread platform.

By including other well service equipment and services such as wirelineunits, coil tubing units or more than two secondary drilling units,further efficiency can be achieved. The method according to theinvention can be used for drilling, completion, intervention, plug andabandonment and other well operations. The space and capacities of themain drilling unit may be limiting the number of simultaneous operationswhich can be possible.

The operating concept of drilling from two drill floors at differentelevations in the same well and on several wells simultaneously canideally be realised by using a jack-up rig as a main drilling rig, butcan alternatively be realised by using an elevated drilling rig on afixed offshore installation or use of a land rig on elevated skid beamsor similar as well as on subsea wells using a mobile offshore drillingrig, drill ship, drill barge or similar positioned above secondarydrilling units, or “drilling pods”, situated below the main drillingrig.

The secondary drilling units can either be operated locally by anoperator working at the secondary drilling unit or remotely from themain drilling rig.

OBJECTS OF THE INVENTION

It is a specific object of the invention to provide a novel method andapparatus utilizing a main drilling rig to support the operation ofseveral well centers. The new idea of using two or more secondarydrilling units optimized for size and weight supported from the maindrilling unit of the main drilling rig with the necessary utilities, andto be able to position the main drilling unit directly above thesecondary drilling units or drilling pods to assist with heavy lifts andlong trips, compensating for the limited capabilities built into thesecondary drilling units. The combination of the secondary drillingunits and main drilling unit will by this obtain a considerably higherefficiency than the traditional drilling rig alone and will be able towork on simultaneous wells located on a relative narrow wellhead spreadarea.

It is another object of the invention to provide a novel method andapparatus for drilling several wells simultaneously by use of smallersecondary drilling units or pods assisted by an elevated high spec maindrilling unit to increase the efficiency of the total operation byperforming tubular operations in the bigger derrick of the main drillingunit and performing other well operations using the smaller secondarydrilling units.

It is a related object of the invention to provide a novel drillingmethod and apparatus where the distributed well centers can be utilizedto reduce the time consequence of equipment breakdown as progress canstill be made with a functional secondary drilling unit or pod or themain drilling unit, and thus enhancing the reliability of the operation.

It is a further object of the invention to provide a novel drillingmethod and apparatus for shallow water drilling from a jack-up where thedistributed well centers can be used to reduce the need to wait for aweather window for reposition the jack-up as the secondary drillingunits or pods may be skidded beyond the reach of the jack-up and be usedto continue well operations as well as tubular operations on such wells.

It is also an object of the invention to provide a novel drilling methodand apparatus for drilling from a fixed offshore installation or landrig where the distributed well centers can be used to reduce time andcost for field development and well decommissioning (Plug &Abandonment).

BRIEF SUMMARY OF THE INVENTION

The system according to the invention is characterised in that aplurality of secondary drilling units, each comprising its own drillingequipment for performing drilling operations, are positioned on thewellhead cluster below the main drilling unit for performing welloperations in the plurality of hydrocarbon wells, wherein the maindrilling unit and at least one of the plurality of secondary drillingunits are arranged for performing alternating well operations in atleast one of the plurality of hydrocarbon wells.

The main drilling unit may have a well operations capacity in at leastone of said well operations which is higher than the well operationscapacity of said at least one of the plurality of secondary drillingunits.

The system may comprise at least one umbilical running from the maindrilling unit to the wellhead cluster for providing drilling support tosaid plurality of secondary drilling units and/or for allowing controland operation of the plurality of secondary drilling units from the maindrilling unit.

The main drilling unit may provide at least one of mud control, powersupply and hydraulics supply to the wellhead cluster.

The main drilling unit may be movable in a plane above the wellheadcluster.

The main drilling rig may be any one of a jack-up rig, a drillingmodule, a drillship, a tension leg platform, a jacket, a land rig or asemi-submersible platform.

The system may be used for distributed well activities in said pluralityof hydrocarbon wells.

The system according to the invention may be used for distributed wellactivities, comprising a main drilling rig with a main drilling unit, adrill floor equipped with one or more well centers, and one or more ofset back/racking area, racking machines, catwalk machinery and ironroughnecks, wherein said main drilling unit is placed on a skiddablecantilever with a drilling mast and drilling machinery, wherein awellhead spread is placed adjacent and below the cantilever of the maindrilling rig, said wellhead spread comprising several secondary drillingunits or pods, where each secondary drilling unit has its own drillingequipment for performing drilling operations, said secondary drillingunits being arranged for simultaneously well operations in separatewells, while being supported by the main drilling unit.

The main drilling rig may comprise a skiddable cantilever comprising adrilling mast and drilling machinery, wherein said main drilling unit isplaced on the skiddable cantilever, wherein the wellhead spread isplaced adjacent and below the cantilever of the main drilling unit, saidwellhead spread comprising said secondary drilling units, where eachsecondary drilling unit has its own drilling equipment for performingdrilling operations, and each secondary drilling unit is arranged forsimultaneously well operations in separate wells while being supportedby the main drilling unit.

The wellhead spread may comprise distributed well centers comprising thesecondary drilling units.

The main drilling rig may be a jack-up rig standing on support legs andcomprising a barge that can be raised or lowered, the jack-up rig beingfurther equipped with a cantilever comprising the main drilling unit,which cantilever can be driven or skidded out over water, such that themain drilling unit is above an operational area to perform desireddrilling operations.

The secondary drilling units may each comprise a mast, drill floor andBOP, which mast comprises a rotary drilling machine, which is adapted todrive a tubular through the BOP and into a desired wellhead.

The main drilling unit may comprises a driller's cabin/control stationwhich is connected by at least one umbilical to the equipment on thewellhead spread for control and operations of the secondary drillingunits and equipment on the wellhead spread.

The main drilling unit may support the wellhead spread with at least oneof mud control, power supply and hydraulics supply.

The method according to the invention is characterised by the steps of:

-   -   positioning a main drilling unit displaying a drill floor and        comprising a drilling and hoisting system operating on the drill        floor above the wellhead cluster,    -   positioning a plurality of secondary drilling units at wellheads        on the wellhead cluster, and    -   alternatingly or simultaneously performing well operations in        the hydrocarbon wells using the main drilling unit and at least        one of said secondary drilling units.

The method may further comprise the steps of:

-   -   positioning the main drilling unit above the wellhead cluster        such that a well center of the drilling unit becomes positioned        vertically above a first wellhead of a first hydrocarbon well of        the wellhead cluster,    -   performing a well operation in the first hydrocarbon well using        the main drilling unit,    -   repositioning the main drilling unit such that the well center        of the drilling unit becomes positioned vertically above a        second wellhead of a second hydrocarbon well of the wellhead        cluster,    -   performing a well operation in the second hydrocarbon well using        the main drilling unit and simultaneously performing a well        operation in the first hydrocarbon well using a first secondary        drilling unit positioned on the wellhead spread at the first        wellhead.

The step of positioning a plurality of secondary drilling units atwellheads on the wellhead cluster may comprise deploying the pluralityof secondary drilling units from a main drilling rig carrying the maindrilling unit.

The method may be realised using a system comprising a main drilling rigcomprising an elevated main drilling unit comprising a drill floorpositioned on a skiddable cantilever, equipped with at least one mainwell center, and a wellhead spread placed adjacent and below thecantilever of the main drilling rig, wherein the method comprises thesteps of:

(a) deploying, from the main drilling rig, several secondary drillingunits onto a well deck of the wellhead spread,

(b) running well operations in a first well from the main well centerthrough a first secondary drilling unit on the wellhead spread,

(c) breaking the connection between the main well center and the firstsecondary drilling unit,

(d) proceeding with drilling operations in the first well using thefirst secondary drilling unit,

(e) moving the main drilling rig to a position above a second secondarydrilling unit on the wellhead spread,

(f) running well operations in a second well from the main well centerthrough the second secondary drilling unit on the wellhead spread,

(g) breaking the connection between the main well center and the secondsecondary drilling unit, and

(h) proceeding with well operations in the second well using the secondsecondary drilling unit.

The main drilling unit may perform preparations for the next activity,such as making up bottom hole assembly or rack up or down casing, whilenot supporting one of the secondary drilling units.

In step (b) the main drilling unit may be arranged for tripping in withbottom-hole assembly through the first secondary drilling unit's rotarybushing and BOP, and when reaching TD the drill string may be landed insaid secondary drilling unit's rotary bushing and an iron roughneckmachine on the secondary drilling unit may be arranged to break theconnection freeing the main drilling unit for other operations.

The main drilling unit may be arranged to move over to the secondsecondary drilling unit immediately after breaking the connectionbetween the main well center and the first secondary drilling unit toassist the second secondary drilling unit while the first secondarydrilling unit is drilling ahead using local pipe storage.

In step (f) the main drilling unit may be arranged for tripping in withthe bottom hole assembly to TD through the second secondary drillingunit, and when reaching TD the string may be landed out in the rotarybushing on the second secondary drilling unit and an iron roughneck maybe arranged to break the connection releasing the drill string from themain well center, while the second secondary drilling unit startsdrilling using its own local drill pipe storage.

The main drilling rig may be arranged to prepare the casing section forthe first secondary drilling unit, move over and, when ready, pull outthe drill string from the first secondary drilling unit and then run inthe casing section through the secondary drilling unit's rotary bushingand BOP.

When the run is close to be completed, the casing landing string may betransferred to the secondary drilling unit which is landing out thecasing.

A preferred embodiment of the invention which is intended to accomplishat least some of the foregoing objects comprises distributing wellactivities to secondary drilling units, or “drilling pods”, to carry outseveral simultaneous drilling or well operations supported from one maindrilling rig, such as a jack-up, drilling module, drillship, tension legplatform, jacket, land rig or semi submersible.

The distributed well activities are obtained by placing the secondarydrilling units on the wellhead platform (wellhead spread), jacket orsimilar in order to carry out well operations such as drilling, reaming,milling and similar from the “drilling pod”. The secondary drillingunits may be supported by umbilicals and service loops from the maindrilling unit in order to provide the necessary power and utilities fromthe main drilling unit to the distributed well centers.

The above mentioned objects are achieved with a system for distributedwell activities, comprising a first, main drilling rig comprising a maindrilling unit comprising a drill floor equipped with one or more of wellcenters, set back/racking area, racking machines, catwalk machinery andiron roughnecks, said main drilling unit is positioned on a skiddabledeck or cantilever, wherein a wellhead spread is placed adjacent andbelow the cantilever or skiddable deck of the main drilling rig, saidwellhead deck comprises several secondary drilling units or pods, whereeach secondary drilling unit or pod has its own drilling equipment forperforming well operations, and each secondary drilling unit or pod isbeing arranged for simultaneous well operations in separate wells, whilebeing supported by the main drilling unit.

Similarly the same can be achieved for subsea wellhead templates byusing a floating main drilling rig comprising the main drilling unit andsuitable secondary drilling units or pods positioned on the subseawellhead template.

The wellhead platform spread may comprise distributed well centers, i.e.drilling pods.

The main drilling rig can be a jack-up rig standing on support legs andcomprising a barge that can be raised or lowered, said jack-up rigfurther being equipped with a cantilever comprising the main drillingunit, where the cantilever can be driven or skidded out over water, suchthat the main drilling unit can be positioned above an operational areato perform desired well operations.

The main drilling rig can alternatively be any other type of structurecomprising an elevated movable drilling unit which is capable of workingabove the secondary drilling units or pods.

The secondary drilling units or pods can each comprise a mast, drillfloor and BOP, and in the mast of the secondary drilling unit there canbe a rotary drilling machine, which is adapted to drive a tubularthrough the BOP and into a desired wellhead.

The main drilling unit can comprise a driller's cabin/control stationconnected by umbilicals to the equipment on the wellhead deck, forcontrol and operations of the secondary drilling units and otherequipment on the wellhead deck.

The main drilling unit can support the secondary drilling units andother equipment on the wellhead deck with all main services andutilities, such as mud, power, hydraulics and similar.

The above mentioned objects are also achieved with a method fordistributed well activities realised by a system comprising a maindrilling rig comprising an elevated main drilling unit positioned on askiddable cantilever equipped with one or several well centers, and awellhead spread positioned adjacent and below the cantilever of the maindrilling rig, wherein the method comprises the steps of: (a) deploying,from the elevated main drilling unit, several secondary drilling unitsor pods onto a well deck of the wellhead spread; (b) running welloperations in a first well from the main drilling unit through a firstsecondary drilling unit or pod on the wellhead spread; (c) breaking theconnection between the main drilling unit and the first secondarydrilling unit; (d) proceeding with drilling operations in the first wellusing the first secondary drilling unit; (e) moving the main drillingunit to a position above a second secondary drilling unit or pod on thewellhead spread; (f) running well operations in a second well from themain drilling unit through the second secondary drilling unit on thewellhead spread; (g) breaking the connection between the main drillingunit and the second secondary drilling unit; and (h) proceeding withdrilling operations in the second well using the second secondarydrilling unit.

The elevated drilling unit can perform preparations for the nextactivity, such as making up bottom hole assembly or rack up or downcasing, while not supporting one of the secondary drilling units.

In step (b), the elevated main drilling unit can be tripping in with abottom-hole assembly through the first secondary drilling unit's rotarybushing and BOP, and when reaching total well depth (TD) the drillstring can be landed in said secondary drilling unit's rotary bushingand an iron roughneck machine on the secondary drilling unit can breakthe connection freeing the main drilling unit for other operations.

Immediately after the connection between the main drilling unit and thefirst secondary drilling unit is broken, the main drilling unit can moveover to the second secondary drilling unit to assist the secondsecondary drilling unit while the first secondary drilling unit isdrilling ahead using local pipe storage.

In step (f), the elevated main drilling unit can be tripping in with thebottom hole assembly to TD through the second secondary drilling unit,and when reaching TD the string can be landed in the rotary bushing ofthe second drilling unit and an iron roughneck can break the connectionreleasing the drill string from the elevated main well center, while thesecond secondary drilling unit starts drilling using its own local drillpipe storage.

The elevated main drilling unit can prepare the casing section for thefirst secondary drilling unit, move over and when ready, pull out thedrill string from the first secondary drilling unit and then run in thecasing section through the secondary drilling unit's rotary bushing andBOP.

When the run is close to be completed, the casing landing string can betransferred to the secondary drilling unit which is landing out thecasing.

DRAWINGS

FIGS. 1 and 2 disclose schematic top-views of a plurality of wellheads101 of wells on which the operational principle of the present inventioncan be employed.

FIG. 3 discloses a top view of a main drilling rig comprising a maindrilling unit.

FIGS. 4 and 5 disclose a system for performing well operations in aplurality of wells according to the invention.

FIG. 6 illustrates a secondary drilling unit or pod for use in theinvention.

FIG. 7 illustrates a jack-up drilling rig for use in the invention.

FIG. 8 illustrates a jack-up drilling rig comprising a main drillingunit and a wellhead platform comprising secondary drilling units or podsfor use in the invention.

FIG. 9 illustrates a drill floor on a main drilling rig.

FIG. 10 shows a comparison between the operation of a standard drillingunit and the distributed well concept according to the invention.

DETAILED DESCRIPTION

In the following, the basic principles of the present invention will bediscussed in more detail with reference to FIGS. 1 to 5.

FIGS. 1 and 2 disclose schematic top-views of a plurality of wellheads101 of wells on which the operational principle of the present inventioncan be employed.

FIG. 1 discloses a plurality of wellheads 101 which are positioned in agrid pattern on a wellhead template 102, as is common for offshoreapplications where the wellhead template 102 is usually located on awellhead platform.

FIG. 2 discloses a plurality of wellheads 101 which are locate within alimited geographical area 103 but which are not positioned on a commonwellhead template. Such a wellhead configuration is sometimes used inland-based or subsea application instead of positioning the wellheads ona common wellhead template, although wellhead templates are sometimesused in land-based or subsea applications as well.

The inventive concept of the present invention is equally applicable toboth of the above-described wellhead configurations, and for the purposeof this specification the term “wellhead cluster” 104 will be used tocomprise any one of a plurality of wellheads which are located on acommon wellhead template, e.g. on a wellhead jacket platform or awellhead spread, or within a limited geographical area withoutnecessarily being located on a common wellhead template.

A plurality of drilling units, or drilling pods 105, is associated withthe wellhead cluster 104. Each drilling unit 105 comprises equipment forperforming well operations or well activities, e.g. drilling, millingand jarring, pumping and cementing and wireline operations. Suchequipment are known in the art and will not be descried in any detailhere, but may typically include a rotary support table, a drilling mastor derrick and a rotary drilling and hoisting machine.

At least one of the drilling units 105 may be movable between differentwellheads 101 of the wellhead cluster 104. If the wellheads 101 arelocated on a wellhead template 102, it may be advantageous to positionthe drilling units 105 on an x-y skid (not indicated in the drawings).

FIG. 3 discloses a schematic top-view of a drilling rig or supportstructure 106 which forms part of the system according to the invention.The drilling rig 106 comprises a drilling unit 107 which is movablymounted on the drilling rig 106 and which comprises a drilling floor ordeck 108 on which a drilling and hoisting system 109 is located. Thedrilling floor 108 displays a well center or well center position 110from which the drilling and hoisting system can perform well operationsor activities.

The drilling unit 107 is movably mounted on the drilling rig 106 suchthat it can be moved or repositioned along the plane of the drillingfloor or deck 108. Such movement or repositioning can be achieved byskidding the drilling unit 107 in orthogonal x and y directions, as isindicated by reference characters X and Y in FIG. 3. However, therepositioning can alternatively be achieved by other movement patterns,e.g. angular rotation and radial skidding.

The drilling unit 107 comprises the equipment and facilities necessaryfor the drilling and hoisting system 109 to perform the well operationsor activities. Within the art, such equipment is known as such and willnot be discussed in any detail here. However, the equipment andfacilities may advantageously comprise a stand building area or position111 where operations can be performed to prepare the drilling andhoisting system 109 for a subsequent well operation, a rail system 112for catwalk machinery, a set back or racking area 113, a racking machine114, iron roughnecks 115 and a driver's cabin or control station 116. Toimplement the actual well operation or activity, the drilling andhoisting system 109 comprises a support table 117, a guide structure 118and a rotary drilling and hoisting machine 119 (cf. FIG. 4).

FIG. 4 is a schematic disclosure of the system according to theinvention. The system comprises a main drilling unit 107 which ismovably mounted on a drilling rig or support structure 106, e.g. ajack-up rig or an offshore platform. The main drilling unit 107comprises a drilling floor 108 on which a drilling and hoisting system109 is arranged to operate centered about a well center 110.

The system also comprises a wellhead cluster 104 which comprises first,second and third wellheads 101 a, 101 b, 101 c of first, second andthird hydrocarbon wells 120 a, 120 b, 120 c. First and second secondarydrilling units 105 a, 105 b are positioned over the first and secondwellheads 101 a and 101 b, respectively. Each secondary drilling unit105 a, 105 b comprises a support table 121 and may advantageously alsocomprise a drilling mast or derrick 122 and a rotary drilling andhoisting machine 123.

The main drilling unit 107 is positioned above the wellhead cluster 104and is movably mounted onto the drilling rig 106 such that the wellcenter 110 can be positioned vertically above any of the wellheads 101a, 101 b, 101 c, allowing the drilling and hoisting system 109 toperform well operations and activities in the hydrocarbon wells 120 a,120 b, 120 c. In other words, the main drilling unit 107 is arrangedsuch that the drilling floor 108 can be moved above the wellhead cluster104. If the wellheads 101 a, 101 b, 101 c are arranged on a wellheadtemplate, it may be advantageous if the main drilling unit 107 isarranged such that the plane defined by the drilling floor 108 isparallel to the plane defined by the wellhead template, i.e. the planein which the wellheads are arranged.

The drilling and hoisting system 109 is supported by the apparatuses,sub-systems and equipment of the main drilling unit 107. Consequently,the main drilling unit 107 will have a well operations capacity which ishigher than the well operations capacity of the secondary drilling units105 a, 105 b. In other words, the drilling and hoisting system 109 canperform well operations faster and more efficiently than the secondarydrilling units 105 a, 105 b. There may also be some well operationswhich the main drilling unit 107 but not the secondary drilling units105 a, 105 b can perform.

According to the method of the invention, each hydrocarbon well 120 a,120 b, 120 c can be operated on alternatingly by the main drilling unit107 and a secondary drilling unit 105 a, 105 b. This will allow welloperations to be allocated to the main drilling unit 107 and thesecondary drilling units 105 a, 105 b in an efficient manner. It willalso allow the main drilling unit 107 to perform well operations in onehydrocarbon well at the same time as at least one of the secondarydrilling unit 105 a, 105 b performs well operations in anotherhydrocarbon well, thus allowing well operations to be performed on atleast two wells at the same time.

A typical sequence of operations according to the inventive method willnow be disclosed in more detail with reference to FIGS. 4 and 5.

The main drilling unit 107 is positioned above the wellhead cluster 104such that the well center 110 becomes positioned vertically above thefirst wellhead 101 a allowing the drilling and hoisting system 109 toperform a first well operation in the first hydrocarbon well 120 a. Inthe illustrated embodiment, the drilling and hoisting system 109performs the first well operation through the support table 121 of thefirst secondary drilling unit 105 a.

When the first well operation is completed, the connection between thedrilling and hoisting system 109 and the first secondary drilling units105 a is broken and the main drilling unit 107 is repositioned such thatthe well center 110 becomes positioned vertically above the secondwellhead 101 b of the wellhead cluster 104.

Once the connection between the drilling and hoisting system 109 and thefirst drilling units 105 a is broken, the first secondary drilling unit105 a is free to perform a second well operation in the firsthydrocarbon well 120 a, and once the drilling and hoisting system 109 isin position above the second wellhead 101 b, and the drilling andhoisting system 109 is free to perform a first well operation in thesecond hydrocarbon well 120 b, as is disclosed in FIG. 5.

Once the first well operation in the second hydrocarbon well 120 b iscompleted, the main drilling unit 107 can be repositioned to anotherwell, or back to the first well 120 a, to perform further welloperations, and the second secondary drilling unit 105 b can proceedwith a second well operation in the second well 120 b.

As discussed above, demanding well operations are advantageouslyallocated to the main drilling unit 107 while less demanding welloperations are allocated to the secondary drilling units 105 a, 105 b.

It may be advantageous if at least one of the secondary drilling units105 a, 105 b can be repositioned between the wellheads such that allhydrocarbon wells on the wellhead cluster can be reached by at least onesecondary drilling unit even if the number of secondary drilling unitsis less than the number of wellheads.

It may be advantageous if the power and utilities necessary to operatethe secondary drilling units 105 a, 105 b are provided from the drillingrig 106 or the main drilling unit 107. For example, it may beadvantageous if such power and utilities are provided via umbilicals andservice loops 124 running from the main drilling unit 107 to thewellhead cluster 104.

In offshore applications, where the wellhead cluster 104 is typicallyarranged on a wellhead template located on a wellhead platform, thedrilling rig 106 may typically be a jack-up rig, a drillship, asemi-submersible platform or a tension leg platform. However, thedrilling structure may be any type of structure which is capable ofmovably supporting a main drilling unit over a wellhead cluster. Themain drilling unit 107 may typically be mounted on a skiddablecantilever, allowing the main drilling unit to be moved betweendifferent hydrocarbon wells of the wellhead cluster.

According to one embodiment of the invention, the drilling structure 106is a jack-up rig.

A jack-up rig or a self-elevating unit is a type of mobile platform thatconsists of a buoyant hull fitted with a number of movable legs, capableof raising its hull over the surface of the sea. The buoyant hullenables transportation of the unit and all attached machinery to adesired location. Once on location the hull is raised to the requiredelevation above the sea surface supported by the sea bed. The legs ofsuch units may be designed to penetrate the sea bed, may be fitted withenlarged sections or footings, or may be attached to a bottom mat. Ajack-up rig is regarded as well known to a skilled person and istherefore not described in detail, but a typical jack-up rig is ingeneral discussed later in the description.

A jack-up is a floating barge fitted with long support legs that can beraised or lowered. The jack-up is maneuvered (self-propelled or bytowing) into location with its legs up and the hull floating on thewater. Upon arrival at the work location, the legs are jacked down ontothe seafloor. Then “preloading” takes place, where the weight of thebarge and additional ballast water are used to drive the legs securelyinto the sea bottom so they will not penetrate further while operationsare carried out. After preloading, the jacking system is used to raisethe entire barge above the water to a predetermined height or “air gap”,so that wave, tidal and current loading acts only on the relativelyslender legs and not on the barge hull.

Modern jacking systems use a rack and pinion gear arrangement where thepinion gears are driven by hydraulic or electric motors and the rack isaffixed to the legs.

Jack-up rigs can only be placed in relatively shallow waters, generallyless than 120 meters (390 ft) of water. However, specialized classes ofjack-up rigs known as premium or ultra-premium jack-ups are known tohave operational capability in water depths ranging from 150 to 190meters (500 to 625 feet).

FIG. 7 is a drawing of a typical jack-up drilling rig 10 suitable foruse together with the distributed well concept to perform effectivetubular operations through a secondary drilling unit or pod and thentransfer the operation back to the secondary drilling unit forperforming slower drilling, milling or similar operations. The jack-uprig comprises a barge 12 fitted with long support legs 14 that can beraised or lowered, wherein said support legs 14 are placed on the seafloor 18. As shown, the barge 12 is located above the sea level 16 at adesired level. The jack-up rig 10 is further equipped with a cantilever20 with a main drilling unit 22, and where the cantilever can be drivenor skidded out over the water, such that the main drilling unit 22 isabove an operational area to perform the desired drilling operations.The operational area is indicated by the two vertical dotted linesrunning down from the cantilever 20. The main drilling unit 22 isequipped in a normal manner with a drilling mast/derrick 24 and drillingmachinery 26. The main drilling unit 22 is operable on a drilling floor28.

FIG. 6 shows a drawing of a typical secondary drilling unit, or“drilling pod”, 30 with mast 32, drill floor 34 and BOP 36. The BOP islocated above the wellhead 42. In the mast 32 of the secondary drillingunit 30 a rotary drilling and hoisting machine 38 is fitted which isadapted to drive a tubular 40 through the BOP 36 and into a desiredwellhead 42. Simple secondary drilling units or “drilling pods” likethis are suitable for use in the distributed well activity concepts. Thesecondary drilling units can also be more advanced if the welloperations so require.

FIG. 8 is a typical setup for the distributed well center operation withtwo secondary drilling units, or “drilling pods”, 30 assisted by ajack-up rig 10. As shown the secondary drilling units 30 are placed on aseparate platform 50 standing on its own legs 54 on the sea floor 18,such as a wellhead platform (wellhead spread). Several secondarydrilling units, or pods, 30, at least two, are placed on the drill floor34 of the auxiliary wellhead platform 50, and can be skidded around forplacement above a desired wellhead 42. A riser 44 can be placed betweenthe BOP 36 and the wellhead 42. From the wellheads 42 several tubulars56 run down into the water and down into hydrocarbon well bores at thesea floor 18. Umbilicals 58 and service loops run from the main drillingunit 22 in order to provide the necessary power and utilities from themain drilling unit to the distributed well centers on the wellheadplatform 50.

FIG. 9 is a typical layout of a drill floor 28 on a jack-up rig 10capable of efficient tubular handling and offline stand building. Asshown, in the upper left corner of the drill floor 28 is a driller'scabin/control station 60 connected to the equipment on the drill floor28 on the cantilever 20. The driller's cabin/control station 60 is alsoconnected by the umbilicals 58 to the equipment on the wellhead platform50, for control and operations of the equipment and the secondarydrilling units 30.

Further, the drill floor 28 is equipped with a set back/racking area 62and, as shown, with two racking machines 64 placed in front of the setback/racking area 62. Running longitudinal on the drill floor 28 are tworails 66 for catwalk machinery. Between the rails 66 is a main wellcenter 68, to the right in the drawing, and an auxiliary stand buildingarea 70, to the left in the drawing, each equipped with an ironroughneck 72. At the end of the rails, the drilling machine 26 of themain drilling unit 22 is shown.

Consequently, the drill floor 28 and the equipment thereon form anelevated main drilling unit 22 which is carried by the cantilever 20,which main drilling unit 22 generally has the same configuration and thesame equipment as the main drilling unit 107 disclosed in FIG. 3.

SEQUENCE OF OPERATION

The sequence of operation and distribution of tasks between the elevatedmain drilling unit 22 and the distributed well centers on the wellheadplatform 50 is set up so that any tripping activities or similar wherethere is a significant efficiency gain to have a tall derrick and largesetback area is performed by the elevated main drilling unit 22. Allslow progressing activities such as drilling ahead, milling, circulatingand so on where there is little difference between a high spec and a lowspec unit can be carried out in the distributed well centers. A typicalsequence is illustrated in FIGS. 8 and 10. Initially the secondarydrilling units 30 are deployed from the main drilling unit 22 and riggedon the well deck of the wellhead platform 50. The operation proceeds bythe main elevated drilling unit 22 tripping in with bottom-hole assemblythrough a first secondary drilling unit's 30 rotary bushing and BOP.When reaching TD the drill string is landed in the secondary drillingunit's rotary bushing and the iron roughneck machine on the firstsecondary drilling unit breaks the connection freeing the main wellcenter 68 of the main drilling unit 22 for other operations. Theelevated drilling unit 22 will then immediately move over to a positionabove the second secondary drilling unit 30 to assist the secondsecondary drilling unit while the first secondary drilling unit 30 isdrilling ahead using local pipe storage. The elevated main drilling unit22 will perform preparations for the next activity such as making upbottom hole assembly or rack up or down casing. The elevated maindrilling unit will trip in with the bottom hole assembly to TD throughthe second distributed well center. When reaching TD the string islanded out in the rotary bushing on the second secondary drilling unitand the iron roughneck breaks the connection releasing the drill stringfrom the elevated main drilling unit. The second secondary drilling unitcan then start drilling using its own local drill pipe storage. Theelevated main drilling unit will prepare the casing section for thefirst secondary drilling unit, move over and when ready pull out thedrill string from the first secondary drilling unit and then run in thecasing section through the secondary drilling unit's rotary bushing andBOP. When the run is close to completed the casing landing string istransferred to the secondary drilling unit, which can land out thecasing.

By using this principle two or more wells can be drilled simultaneouslyfrom one main drilling unit by use of two or more distributed wellcenters keeping the well control for the well at all times. The size ofthe distributed well centers can be kept small as the main drilling unitis supplying services including mud, power, hydraulics and so on.Furthermore the main drilling unit can be utilized to time optimizeoperations or operations requiring increased capabilities such astripping in and out of the hole and running or pulling casings.

1. A system for performing well operations in a plurality of hydrocarbon wells of a wellhead cluster, which system comprises a main drilling rig comprising a main drilling unit displaying a drill floor and comprising a drilling and hoisting system arranged to operate on the drill floor about a well center, wherein the main drilling unit is positioned at least partially vertically above the wellhead cluster enabling the drilling and hoisting system to perform well operations in the plurality of hydrocarbon wells, wherein a plurality of secondary drilling units, each comprising its own drilling equipment for performing drilling operations, are positioned on the wellhead cluster below the main drilling unit for performing well operations in the plurality of hydrocarbon wells, wherein the main drilling unit and at least one of the plurality of secondary drilling units are arranged for performing alternating well operations in at least one of the plurality of hydrocarbon wells.
 2. The system according to claim 1, wherein the main drilling unit has a well operations capacity in at least one of said well operations which is higher than the well operations capacity of said at least one of the plurality of secondary drilling units.
 3. The system according to claim 1, further comprising at least one umbilical running from the main drilling unit to the wellhead cluster for providing drilling support to said plurality of secondary drilling units and/or for allowing control and operation of the plurality of secondary drilling units from the main drilling unit.
 4. The system according to claim 3, wherein the main drilling unit provides at least one of mud control, power supply, hydraulics supply to the wellhead cluster.
 5. The system according to claim 1, wherein the main drilling unit is movable in a plane above the wellhead cluster.
 6. The system according to claim 1, wherein the main drilling rig is any one of a jack-up rig, a drilling module, a drillship, a tension leg platform, a jacket, a land rig or a semi-submersible platform.
 7. The system according to claim 1, wherein said plurality of hydrocarbon wells comprise distributed well activities.
 8. The system according to claim 1, wherein said main drilling rig comprises a skiddable cantilever comprising a drilling mast and drilling machinery, wherein said main drilling unit is placed on the skiddable cantilever, wherein the wellhead spread is placed adjacent and below the cantilever of the main drilling unit, said wellhead spread comprising said secondary drilling units, where each secondary drilling unit has its own drilling equipment for performing drilling operations, and each secondary drilling unit is being arranged for simultaneously well operations in separate wells while being supported by the main drilling unit.
 9. The system according to claim 8, wherein the wellhead spread comprises distributed well centers comprising said secondary drilling units.
 10. The system according to claim 8, wherein the main drilling rig is a jack-up rig standing on support legs and comprising a barge that can be raised or lowered, the jack-up rig being further equipped with the cantilever supporting the main drilling unit, where the cantilever can be driven or skidded out over water, such that the main drilling unit is above an operational area to perform desired drilling operations.
 11. The system according to claim 8, wherein the secondary drilling units each comprises a mast, drill floor and BOP, which mast comprises a rotary drilling machine which is adapted to drive a tubular through the BOP and into a desired wellhead.
 12. The system according to claim 8, wherein the main drilling unit comprises a driller's cabin/control station connected by at least one umbilical to the equipment on the wellhead spread for control and operations of the secondary drilling units and equipment on the wellhead spread.
 13. A method of performing well operations in a plurality of hydrocarbon wells of a wellhead cluster, comprising the steps of: positioning a main drilling unit displaying a drill floor and comprising a drilling and hoisting system operating on the drill floor above the wellhead cluster, positioning a plurality of secondary drilling units at wellheads on the wellhead cluster, and alternatingly or simultaneously performing well operations in the hydrocarbon wells using the main drilling unit and at least one of said secondary drilling units.
 14. The method according to claim 13, comprising the steps of: positioning the main drilling unit above the wellhead cluster such that a well center of the drilling unit becomes positioned vertically above a first wellhead of a first hydrocarbon well of the wellhead cluster, performing a well operation in the first hydrocarbon well using the main drilling unit, repositioning the main drilling unit such that the well center of the drilling unit becomes positioned vertically above a second wellhead of a second hydrocarbon well of the wellhead cluster, performing a well operation in the second hydrocarbon well using the main drilling unit and simultaneously performing a well operation in the first hydrocarbon well using a first secondary drilling unit positioned on the wellhead spread at the first wellhead.
 15. The method according to claim 13, wherein the step of positioning a plurality of secondary drilling units at wellheads on the wellhead cluster comprises deploying the plurality of secondary drilling units from a main drilling rig carrying the main drilling unit.
 16. The method according to claim 13, wherein the method is realised using a system comprising a main drilling rig comprising an elevated main drilling unit comprising a drill floor positioned on a skiddable cantilever, equipped with at least one main well center, and a wellhead spread placed adjacent and below the cantilever of the main drilling rig, wherein the method comprises the steps of: (a) deploying, from the main drilling rig, several secondary drilling units onto a well deck of the wellhead spread, (b) running well operations in a first well from the main well center through a first secondary drilling unit on the wellhead spread, (c) breaking the connection between the main well center and the first secondary drilling unit, (d) proceeding with drilling operations in the first well using the first secondary drilling unit, (e) moving the main drilling rig to a position above a second secondary drilling unit on the wellhead spread, (f) running well operations in a second well from the main well center through the second secondary drilling unit on the wellhead spread, (g) breaking the connection between the main well center and the second secondary drilling unit , and (h) proceeding with well operations in the second well using the second secondary drilling unit.
 17. The method according to claim 16, wherein the main drilling unit performs preparations for the next activity, such as making up bottom hole assembly or rack up or down casing, while not supporting one of the secondary drilling units.
 18. The method according to claim 16, wherein the main drilling unit in step (b) is tripping in with bottom-hole assembly through the first secondary drilling unit's rotary bushing and BOP, and when reaching TD the drill string is landed in said secondary drilling unit's rotary bushing and an iron roughneck machine on the secondary drilling unit breaks the connection freeing the main drilling unit for other operations.
 19. The method according to claim 16, wherein the main drilling unit moves over immediately to the second secondary drilling unit, after breaking the connection between the main well center and the first secondary drilling unit, to assist the second secondary drilling unit while the first secondary drilling unit is drilling ahead using local pipe storage.
 20. The method according to claim 16, wherein the main drilling unit in step (f) is tripping in with the bottom hole assembly to TD through the second secondary drilling unit, and when reaching TD the string is landed out in the rotary bushing on the second secondary drilling unit and an iron roughneck breaks the connection releasing the drill string from the main well center, while the second secondary drilling unit starts drilling using its own local drill pipe storage.
 21. The method according to claim 16, wherein the main drilling rig prepares the casing section for the first secondary drilling unit, moves over and when ready pulls out the drill string from the first secondary drilling unit and then run in the casing section through the secondary drilling unit's rotary bushing and BOP.
 22. The method according to claim 21, wherein when the run is close to be completed, the casing landing string is transferred to the secondary drilling unit which is landing out the casing. 